US3171967A - Phase current balancing for multiphase rectifier circuits - Google Patents

Description

March Z, 1965 J. scHAEFER 3,171,967

PHASE CURRENT BALANCING FOR MULTIPHASE RECTIFIER CIRCUITS Filed Oct. 3, 1960 3 Sheets-Sheet l March 2, 1965 J. scHAEl-'ER 3,171,967

' PHASE CURRENT BALANCING FOR MULTIPI-IASE RECTIFIER CIRCUITS Filed Oct. 5, 1960 .3 Sheets-Sheet 2 FI-5. E.

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March 2, 1965 J. scHAEFl-:R 3,171,967

PHASE CURRENT BALANCING FOR MULTIPHASE RECTIFIER CIRCUITS Filed oct. s, 1960 s sheets-sneer s United States Patent O 3,171,967 PHASE CURRENT BALANCING FOR MULTi- PHASE RECTIFIER CIRCUiTS Johannes Schaefer, Philadelphia, Pa., assigner to I-T-E Circuit Breaker Company, Philadelphia, Pa., a corporation of Pennsylvania Filed Oct. 3, 1960, Ser. No. 60,186 6 Claims. (Cl. 307-14) This invention relates to a novel control circuit for balancing the phase currents between the various phases of multiphase rectifier systems.

Because of manufacturing tolerances, particularly for very large current rating rectifiers, it is extremely difficult and expensive to have each phase of a rectifier system have the same reactance as the other phases. These dfferences occur, for example, because of different reactances in unsymmetrioally arranged transformer coils of a rectifier transformer, in the bus bars between the A.C. and D.-C. systems, as well as within the rectifier proper.

Once there is a given unbalance between the phases, the total unbalance will increase with an increase in cur rent through the rectifier. At the present time there are several methods used to correct this substantially inherent unbalance between phase currents. For example, there have been attempts to use regulating transformers in the A.-C. circuit. This has not proven too successful generally because a continuous voltage control is not available with regulating transformers, and in some cases to achieve proper current balance an extremely fine control of input A.C. voltage is required.

Other attempts, shown in copending application Serial Number 726,214, filed April 3, 1958, now Patent No. 3,042,849, in the name of I. K. Dortort, assigned to the assignee of the present invention is to provide reactors for each phase of variable iron cross-section. This system, while quite effective, requires shut down of the system and trial and error restacking of the reactor coil. It will be seen that the device of the present invention is adjustable under load. In this solution, however, the transformers become extremely expensive, and are quite large, since they must carry the full powerto be added to the circuit.

The essence of the present invention is to provide a magnetic core in each phase wherein the flux level of the core has a controllable number of volt seconds which must be supplied by the phase prior to the time that the phase is to conduct. Thus, in rectifier circuits where a given phase will have a certain interval during which no current willL be conducted, a core is provided with a bias such that a predetermined number of volt seconds are reversed during a preceding current carrying interval before the current goes to zero. The remaining volt seconds must then be reversed prior to initiation of conduction of that phase for its next current carrying interval so that there will be a delay in the initiation of conduction of the phase. This delay, which is controllable in magnitude by controllingv the amount of volt seconds remaining to be reversed in the core, can be changed for each phase individually so that the net average current passed by each phase is adjustable for an infinitely small number of steps.

In a preferred embodiment of the invention, the bias for the core is derived from the phase current of an adjacent phase whereby an automatic correction is caused so that there will be a constant balance after an initial adjustment regardless of the current passed by the phase in question.

These control circuits will be seen to require very little control power which is essentially negligible as compared to the power being utilized by the rectifier load, and are extremely simple and economical in construction.

Furthermore, since the control circuit power can be 3,171,967 Patented Mar. 2, 1965 ICC derived from the main circuit, it is made independent of the voltage of the circuit and proportional to the current, since the imbalance varies with the current being carried.

Accordingly, a primary object of this invention is to provide a novel control circuit for controlling the phase currents of a multiphase rectifier system.

Another object of this invention is to balance the current between the various phases of a multiphase rectifier system by using a small magnetic core for at least some of the phases which is controlled to cause more or less phase shift of the point at which the phase begins to conduct current.

A further object of this invention is to provide a novel phase current control system for rectifiers wherein the amount of correction provided is a function of the rectifier current and is independent of the rectifier voltage.

These and other objects of this invention will become apparent from the following description when taken in connection with the drawings in which:

FIGURE 1 schematically illustrates a three-phase bridge connected rectifier system which is provided with my novel phase current balancing circuit.

FIGURE 2 illustrates the phase current for each of the three phases of FIGURE 1 as a function of time.

FIGURE 3 illustrates the control current for the control core of FIGURE 1 when plotted on the same time scale as used for FIGURE 2.

FIGURE 4 shows the magnetization curve of any of the control cores of FIGURE l.

FIGURE 5 graphically represents the manner in which the control current is caused to be a direct function of phase current, notwithstanding inaccuracies in the reactors of the control circuit.

FIGURE 6 illustrates the control current as a function of time plotted to the same scale as are FIGURES 2 and 3 for the case of a variableresistor and a reactor in the control circuit rather than a reactor alone, as shown in FIGURE 1.

Referring now to FIGURE 1, I have shown a typical three-phase bridge connected rectifier wherein a power transformer secondary winding 10 is energized from a primary circuit (not shown) and has its secondary conductors 11, 12 and 13 extending to the rectifier bridge. More specifically, the conductor 11 which is a bus bar for the case of extremely high current capacities, is connected to the junction of the bridge arms, including rectifier elements 14 and 15; bus bar 12 is connected to the junction of rectifier arms 16 and 17; and bus bar 13 is connected with the junction between the rectifier arms, including rectifiers 13 and 19.

The positive ends of rectifiers 14, 16 and 18 are connected together and are brought out through a positive bus to positive terminal 20, while the negative ends of rectifiers 15, 17 and 19 are connected together and are brought out through negative terminal 21.

It will be understood .that rectifier-s 14 through 19 may be of any type and could, for example, represent individual .groups or series and parallel connected silicon or germanium rectifier cells connected in a manner described, for example, in copend-ing application Serial No. 628,- 324, filed December 14, 1956, in the name of Isadore K. Dortort, and assigned to the assignee of Ithe present invention, now Patent No. 2,944,028, or as shown in U.S. Patent No. 2,932,781 to Otto Jensen.

In accordance with the present invention, the current through buses 11, 12 land 13 can be balanced through a novel control circuit. The control circuit includes reactor cores 22, 23 .and 24 for buses 11, 12 and 13. Each of reactor cores 22, 23 and 24 hais a relatively small cross-sectional area and is preferably formed of a grain oriented steel. The buses 11, 12 and 13 can pfass straight kactor 24 in accordance with the invention. fclosed circuit is Lprovided for reactor 22 of bus bar 11 through vcores 22, 23 and 24, respectively, to form reactors having a single turn and `thus provide an extremely compact and inexpensive unit.

Each of cores 22, 23 and 24 isso designed as to have ,an extremely small step by virtue of a rela-tively small -- Reactors 25, 26 and 27 vhave a much smaller-volt second rating than the reactors 22, 23 or 24, respectively, and are provided with air gaps 28, 29 and 36, respectively.

Further reactor means 31, 32 and 33 arethen provided ?for the circuit-to -be described, which have adjustable air ` gaps 34, 35 and 36 which can beadjustedby maintenance uvvorkers'to achieve ultimate balancing betweenfphase cur rents in themannerto be described.

The adjustalble air gap structure is not described and could be done in any well known manner. It will be noted that in a further embodiment ofthe invention resistors could be connected in series with the reactors.

An electrical circuit is then Vformed as a closed series circuit which includes a Winding 37 of reactor 2S, winding 38 of reactor 31, and control winding 39 of reactor 24. This .circuit operates to supply control current for re- A similar Referring rst to FIGURE 2, I have shown the phase currents in, i12 and 1'13 for the currents in conductors 11, 12 and 13, respectively,ot FIGURE 1. In the absenceof ya reactor such as reactors 22, 23 and 24, `the currents such as current in will commutate with the cur- `rent i12, and conductor 1l will carry zero current for a Yperiod of somewhat less than 60,

That is-to say, the current 'in at time t2 begins to drop toward zero and would normally Ibe lretained at a zero vlalue until time t4 When the current begins to rise in the negative direction.

`In accordance with the present invention, cores 22, 23 and 24 are provided which are biased by the current 4flowing in windings 42, 45 and 39, respectively, so that during the part of the `60" interval `in which no current flows and as shown in FIGURES 4 and 5, the net ampere turns of conductor 1I passing through core 22 will be at the value z'c which represents the ampere turns gen- ,.erated bywinding'itl. As soon as the sum of the ampere turns in conductor 11 and in winding 42 equals the magnetizing current of `core 22, core 22 will unsaturate and ,1go through a ilux change ifor a time Az*a illustrated in FIGURES 2, 3 and 4. Note that the current scales in FIGURES 2 and 3 are not identical andthat the current c is only a small fraction of the maximum current whichows in conductor 11. This causes only la partial reversal of the ilux of core 22 which stops at the point the actual current conductor 11 is zero. rIlhe current remains at zero until timer@ at which time the current in conductor in would normally begin to conduct in the negative direction. Since, however, before this current conduction can take place there must be additional reversal olf flux of core 22, for an `additional time Arb there is no appreciable current conduction until after the interval tb is over and core 22 is completely saturated. This causes a phase shift in the current in whereby the average current of any half wave may be decreased by increasing the interval Alb. The interval Arb can in turn be increased merely by decreasing the bias current ic.

Conversely, the average current passed can be increased by increasing the current ic. The ellect of kcore 22 can, in fact, be removed completely from the circuit by making the lbias z'c so large that there will be complete flux reversal oi the core during the normally quiescent or nonconductive interval. Therrraximum elect of the core will be achieved when ,the .Ibias current 'ic is zero so that the full ilux ofthe core must be reversed after the quiescent60 interval of cur-rent in conductor lll.

By using'this novel Vphase shitt at the beginning of commutation of Vthe phase currents by ya simple control of bias current in, it .will be kapparent .that the individual bias current can be adjusted individually of one another.

Thus, ythe bias current ic for .the phase includingcon- -ductor 11 is derived from the reactor 26 which will generate a voltage on its winding 40 during vthe interval that current i12 reversed from i-ts maximum to its mini- ,rnumva-lue. However, andfbecause-oif thereactor for-med by core '32 and Winding 4I, the .current z' continues to flow with a decreasing magnitude having a desired value at time t3.

Accordingly, by .adjusting the kair gap 35 of core 32, the impedance of the circuit supplying volt seconds to winding 42 can `be controlled so that the bias for core 22 may be controlled independently of cores 23 and 24. In a like manner, core 23 `is controlled from the phase .includingconductor ,13 whereby adjustment ot air gap 36 ot reactor core 33 will control the bias current ic for Core 23.

Finally, the characteristic of core 24 is controlled from the phase includingconductor l1 .through adjustablereactor 3l.

During these adjustments, and referring speciiically to FIGURE 3, a relativelylarge current 1'C is shown in solid lines for the current suppliedto winding 42 from reactor 26. ,In order todecrease this current as described above, the air gap 35 of `core 32 is varied to change the inductance of winding 41 and-thus decrease the current ic to the dotted line value of FIGUR-E 3 which has substantially the same shape -a's the higher current ualue but a lower magnitude.

Byrusing control circuits ofthe type shown in FIGURE 1, the control current iC will be caused to .vary directly with the main phase current.

Furthermore, by designing Yreactors 25, 26V and 27 so that they havethe same voltrsecond rating as reactors L31,

32 and 33, the non-linearities of the cores are compensated by one-anothersothat the change in control current Will be almost exactlyproportional'tothe change in phase current.

Referringto-FIGURE 5 which schematically illustrates this operation, phase current i12 is plotted against time as a vertical axis. The effect of current z'lgon reactor 26 is schematically illustrated .by the hysteresis loop 50 of reactor 26 which shows the ux of the reactor in the vertical axis and the magnetizing current of the reactor on a horizontal axis. The excursion of flux in reactor 50 isrelated to the volt -second value 51 shown adjacent the hysteresis loop 50 for reactor core k26 where value 51 `represents an envelope of the volt-seconds change in core 32 as a function of time yplotted on a horizontal axis. The volt seconds generated in reactor 26 is equal to the volt seconds absorbed-by reactor 32, disregarding the small amount of volt seconds'generated -in winding 42 during llux reversal of core 22, as pointed out above, so that the voltage applied to Winding 41 Willbe relatedvto the hysteresis loop 52 of reactor core 32 shown atthe right-hand Side of FIGURE 5 which illustrates the ilux lof reactor core 32 on a vertical axis and the current through Winding 41 on a horizontal axis.

The control current can now be derived from hysteresis `loop SZand is shown at-the lower yright-hand corner of 'FIGURE A5 as control current ic.

.between the magnitude of the phase current i12 and the magnitude of control current ic. n

In order to reduce the current ic, the air gap 35 of `reactor core 32 is increased so that the hysteresis loop of core 32 becomes that shown in dotted lines 53. As a result of this change, the control current ic will assume the dotted value shown which is reduced in magnitude although is still substantially the same in Wave shape as is the current i12.

Accordingly, with the novel circuit there is a direct relationship between phase current and control current so that there is automatically controlled regulation of the phase current of all the phases so that a constant relationship between them will remain. This means that a constant balance will be achieved regardless of the magnitude of phase current.

Although I have illustrated the control elements of the various circuits as only reactors 31, 32 and 33, adjustable resistors can be connected in series with these reactors. With the adjustable resistor, the Wave shape changes as shown in FIGURE 6. However, the control current and load current are still in constant relation with one another.

When a respective resistor is connected in series with the reactors 31, 32 and 33, the magnitude of current ic at time t3 is still a function of the a-ir gap of the respective reactor and is shown by curves 54 of FIGURE 6. Where control is achieved by such variable resistors, the bias for phase 11 should come from phase 13 rather than phase 12 since the resistor control will be strongest at time t5 rather than at time t4.

It will be noted that while I have illustrated the invention as having the control reactors and their control elements in the secondary side of the rectiiier, they could be applied in the primary side as Well. Furthermore, the invention is not necessarily limited to balancing between rectilier phases and can be applied to any application dealing with unbalanced currents in adjacent conductors of a multi-phase system so long as there is a time interval of no current flow for the phases.

In the foregoing the invention has been described solely in connection with specific illustrative embodiments thereof. Since many variations and modiications of the invention will now be obvious to those skilled in the art, it is preferred to be bound not by the specific disclosures herein contained but only by the appended claims.

I claim:

1. A phase current balancing circuit for multi-phase systems; said phase current balancing circuit including respective reactor cores for each of said phases; each of said reactor cores having a main winding connected in series with its said respective phase; each of said reactor cores having a biasing winding and an independent source of biasing current connected to each said biasing winding; said source of biasing current for each of said phases being connected to the other of said phases which conducts current before said phase associated with said source of biasing current; said source of biasing current being controlled to reverse a controlled portion of flux of its said respective magnetic core prior to zero passage of current through the said phase associated with said source of biasing current; the remainder of said fluX of said magnetic core being reversed upon change of current in said phase from zero current to provide a controlled time delay in the beginning of current ow from a zero value; and means for adjusting the magnitude of the biasing current for each phase independently of one another to control the amount of flux reversed in each respective core prior to current zero in each respective phase; said magnetic cores having a relatively low magnetizing current.

2. A phase current balancing circuit for multi-phase systems; said phase current balancing circuit including respective reactor cores for each of said phases; each of said reactor cores having a main Winding connected in series with its said respective phase; each of said reactor cores having a biasing winding and an independent source of biasing current connected to each said biasing winding; said source of biasing current for each of said phases being connected to the other of said phases which con- `ducts current before said phase associated with said source of biasing current; said source of biasing current being controlled to reverse a controlled portion of flux of its said respective magnetic core prior to zero passage of current through the said phase associated with -said source of biasing current; the remainder of said flux of said magnetic core being reversed upon change of current in said phase from zero current to provide a controlled time delay in the beginning of current ow from a zero value; and means for adjusting the magnitude of the biasing current for each phase independently of one another to control the amount of flux reversed in each respective core prior to current zero in each respective phase; said magnetic cores being of grain oriented steel.

3. A phase current balancing circuit for multi-phase systems; said phase current balancing circuit including respective reactor cores for each of said phases; each of said reactor cores having a main winding connected in series with its said respective phase; each of said reactor cores having a biasing Winding and an independent source of biasing current connected to each said biasing winding; said source of biasing current for each of said phases being connected to the other of said phases which conducts current before said phase associated with said source of biasing current; said source of biasing current being controlled to reverse a controlled portion of iiux of its said respective magnetic core prior to zero passage of current through the said phase associated with said source of biasing current; the remainder of said iiux of said magnetic core being reversed upon change of current in said phase from zero current to provide a controlled time delay in the beginning of current iow from a zero value; and means for adjusting the magnitude of the biasing current for each phase independently of one another to control the amount of liux reversed in each respective core prior to current zero in each respective phase; said main winding for each of said cores comprising a straight thru portion of a conductor ofthe respective phase.

4. A phase current balancing circuit for multi-phase systems; said phase current balancing circuit including respective reactor cores for each of said phases; each of said reactor cores having a main winding connected in series with its said respective phase; each of said reactor cores having a biasing winding and an independent source of biasing current connected to each said biasing Winding; said source of biasing current for each of said phases being connected to the other of said phases which conducts current before said phase associated with said source of biasing current; said source of biasing current being controlled to reverse a controlled portion of flux of its said respective magnetic core prior to zero passage of current through the said phase associated with said source of biasing current; the remainder of said flux of said magnetic core being reversed upon change of current in said phase from zero current to provide a controlled time delay in the beginning of current flow from a zero value; and means for adjusting the magnitude of the biasing current for each phase independently of one another to control the amount of flux reversed in each respective core prior to current zero in each respective phase; said magnetic cores having a relatively low magnetizing current; said sources of biasing current for each of said phases deriving energy from an adjacent phase.

5. A phase current balancing circuit for multi-phase systems; said phase current balancing circuit including respective reactor cores for each of said phases; each of said reactor cores having a main winding connected in series with its said respective phase; each of said Vreactor cores `having a biasing Winding and an independent-source of biasing currentconnected to each said biasingvvinding; said source of -biasing'current for each -of said phases being connected to the other of said phases which conducts currentbefore said phase associated Wit'h-saidsource ofbiasing current; said source of biasing current being controlled to reverse a controlled portion-of iiux of its said respective magnetic core priorrto -zero ipassage of current through the said phase associated with -saidsource of biasing current; the remainder of -said ux of said magnetic core being reversed upon change of current in said phase from Zero 4current to provide a controlled time delay Vin the Vbeginning of current ow from a zero value; and means vfor adjusting the magnitude of the biasing current for each phase independently of one another to control ,the amount of flux reversed in each yrespective core prior to current zero in each respective phase; said magnetic cores havingairelatively low magnetizing current; said sources of biasing current being electrically connected t-o their said .respective 'biasing windings in series with a respective .reactor having an adjustable air gap therein.

6. Arphase current balancing circuit for multiwphase systems; said phase current balancing circuit including respective yreactor cores for each of Asaid phases; each of said reactor cores having a main Windingconnected in series With its said respective phase; each of said reactor cores havinga biasing windingand an independent source of biasing lcurrent connected to each said biasing winding;

value; and means for adjustingthe magnitude of the biasing current tfor each :phase independently of `one another `to control Vvthe amount of -iiux reversed in each respective core prior yto .current zero in each `respective zphase; said magnetic cores having a relatively low mag- -netizing current; said sources of biasing current being electrically connected to their said respective ybiasing windings in series with a respective reactor having an adjustable air gap'therein; said sources of biasing.current including magnetic reactors having a volt second rating equal -to the volt second rating of their said respective reactors.

References Cited by the Examiner UNITED STATES PATENTS 2,289,090 7/'42 -Bedford 321--26 LLGYD MCGOLLUM, Primary Examiner.

GRIS L. KADER, Examiner.

Claims (1)

1. 5. A PHASE CURRENT BALANCING CIRCUIT FOR MULTI-PHASE SYSTEMS; SAID PHASE CURRENT BALANCING CIRCUIT INCLUDING RESPECTIVE REACTOR CORES FOR EACH OF SAID PHASES; EACH OF SAID REACTOR CORES HAVING A MAIN WINDING CONNECTED IN SERIES WITH ITS SAID RESPECTIVE PHASE; EACH OF SAID REACTOR CORES HAVING A BIASING WINDING AND AN INDEPENDENT SOURCE OF BIASING CURRENT CONNECTED TO EACH SAID BIASING WINDING; SAID SOURCE OF BIASING CURRENT FOR EACH OF SAID PHASES BEING CONNECTED TO THE OTHER OF SAID PHASES WHICH CONDUCTS CURRENT BEFORE SAID PHASE ASSOCIATED WITH SAID SOURCE OF BIASING CURRENT; SAID SOURCE OF BIASING CURRENT BEING CONTROLLED TO REVERSE A CONTROLLED PORTION OF FLUX OF ITS SAID RESPECTIVE MAGNETIC CORE PRIOR TO ZERO PASSAGE OF CURRENT THROUGH THE SAID PHASE ASSOCIATED WITH SAID SOURCE OF BIASING CURRENT; THE REMAINDER OF SAID FLUX OF SAID MAGNETIC CORE BEING REVERSED UPON CHANGE OF CURRENT IN SAID PHASE FROM ZERO CURRENT TO PROVIDE A CONTROLLED TIME DELAY IN THE BEGINNING OF CURRENT FLOW FROM A ZERO VALUE; AND MEANS FOR ADJUSTING THE MAGNITUDE OF THE BIASING CURRENT FOR EACH PHASE INDEPENDENTLY OF ONE ANOTHER TO CONTROL THE AMOUNT OF FLUX REVERSED IN EACH RESPECTIVE CORE PRIOR TO CURRENT ZERO IN EACH RESPECTIVE PHASE; SAID MAGNETIC CORES HAVING A RELATIVELY LOW MAGNETIZING CURRENT; SAID SOURCES OF BIASING CURRENT BEING ELECTRICALLY CONNECTED TO THEIR SAID RESPECTIVE BIASING WINDINGS IN SERIES WITH A RESPECTIVE REACTOR HAVING AN ADJUSTABLE AIR GAP THEREIN.

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